Proper Ferroelastic Transition in Piezoelectric Lithium Ammonium Tartrate

Dielectric and Brillouin scattering experiments on a lithium ammonium tartrate crystal have shown that the transition in this crystal is described by the free energy of a piezoelectric crystal, F=1/2(χˣ)^-1P²+aPx+1/2β(T-T₀)x², where the primary order parameter is the homogeneous strain, which gives rise to ferroelectricity through the piezoelectric coupling with the polarization.